Present equipment for heat transfer between fluids in industry is based on tubular heat exchange equipment in which one fluid passes through a tube and a second fluid passes around the outside of the tube with heat transfer through the tube wall. Most heat transfer requirements are sufficiently large that many tubes are needed and which, typically, operate in parallel. The tubes are contained between tube sheets in a heat exchanger shell with the fluid outside the tubes being directed in such a way that significant turbulence is generated to facilitate heat transfer to the tube wall.
Use of heat exchangers for liquid to liquid heat transfer is illustrated in the manufacture of sulphuric acid as described in U.S. Pat. No. 4,495,161--Gordon M. Cameron et al. issued Jan. 22, 1985, U.S. Pat. No. 4,547,353--Gordon M. Cameron, issued Oct. 15, 1985 and U.S. Pat. No. 4,654,205--Gordon M. Cameron, issued Mar. 31, 1987, wherein hot, concentrated sulphuric acid is cooled by water or a heat transfer fluid, optionally, subsequent to water dilution thereof.
Use of heat exchangers for gas-to-gas heat transfer, particularly, for waste heat recovery is described in "SULPHUR" entitled "Waste Heat Recovery from Sulphuric Acid Plants", Dahya Bhaga, March/April, 1980 published by The British Sulphur Corporation Limited, London, England.
Classically, the key to good exchanger design lies in the most effective use of the available pressure difference between inlet and outlet ends of the shell space to generate heat transfer enhancing turbulence, compatible with appropriate shell construction to give adequate support to the tubes between the tube sheets and to prevent tube vibration and subsequent mechanical damage to the tubes. Baffles are a general means of directing fluid flow and can be arranged to give ordinary cross-flow across the tubes, partial cross-flow in a double segmental design, and an inflow-outflow arrangement referred to as "disc-and-donut" baffling.
Typical heat exchangers use tubes arranged in a variety of cross-sectional layouts, either in in-line or staggered layouts, diagrammatically represented as equilateral triangles, isosceles triangles, square and rotated square pitches and, more recently, a radially symmetrically pitch in which tubes are arrayed in concentric rings with an open core and an open outer annulus. An array of tube layouts in heat exchangers can be seen in Perry, J. H., "Chemical Engineers Handbook", U.S. Pat. No. 4,357,991--Gordon M. Cameron, issued Nov. 9, 1982 and U.S. Pat. No. 5,044,431--Gordon M. Cameron, issued Sep. 3, 1991.
The heat transfer area required in a heat exchanger is normally proportional to the fluid flow and the number of tubes contained in a shell will rise proportionally to the cross-sectional area of the shell. Baffles are normally used to force the shell fluid to flow from one side of the shell to the other side. The open space between the tubes for flow across the tubes is then proportional to the diameter of the shell which is proportional to the square root of the cross-sectional area. The velocity with which the shell side fluid flows across the tubes is also limited by pressure loss and tube vibration conditions. As a result, with larger units, the baffles which direct the cross-flow (and protect the tubes against damage from vibration) must be moved farther and farther apart as the exchanger gets larger to compensate for the difference between the linear growth of the flow and the shell growth as the square root of the area and flow. The other alternative is to move the tubes farther apart so that there is more room for fluid flow. This alternative leads directly to much larger equipment which is more costly and may also be much harder to obtain.
To reduce potential problems caused by fluid flow, several baffle arrangements have been utilized. These include double segmental baffles, where the fluid only flows across half of the tube bundles, steadying baffles located between flow-directing baffles, and more recently re-introduction of the disc-and-donut baffles in which the area of gas flow across the bundle is significantly larger than for simple cross-flow. Problems due to fluid flow are especially significant in large atmospheric pressure gas-to-gas heat transfer equipment. Use of the correct baffle spacing to keep velocity flow at a reasonable level can require baffle spacing as low as 10 cm in a small unit and up to 3 m in a large heat exchanger.
While many units have a first fluid colder at all times than the second fluid, in many other exchangers there is a temperature cross such that the first fluid leaves the exchanger at a temperature above that of the exiting second fluid. For this arrangement to be practical, the fluids must travel in essentially opposite directions. Since a flow pass across the tubes is not counter-current, the greater the number of cross-flow passes through a tube bundle the more closely the flow will approach the counter current case and the more effective will be the heat transfer.
Good shell flow arrangements in large units need to have adequate baffles to generate heat transfer inducing turbulence, good tube support and provide reasonable and sufficient counter-current flow. Further, such designs must also result in reasonable economic costs commensurate with size.
The tube bundle within the shell space may also be spread uniformly across the shell in a design referred to as a "filled shell". In an alternative layout, the tubing may be confined to only part of the exchanger shell to provide open spaces called "windows". These windows are frequently used to transfer the shell-side fluid from one cross-flow pass to the next cross-flow pass and provide designs referred to as "No Tube In Window" (NTIW) designs.
Classically, tube bundles are laid out in straight lines along the length of the shell wherein fluid flows across or through the straight line tube bundle. With fluid flow in a radial direction the shell circumference increases as the radius of the shell increases. One successful method by which the tube bundle is laid out is described in U.S. Pat. No. 4,357,991--Cameron, wherein the tube bundle is laid out in an array of a plurality of circles such that the tubes are staggered to provide maximum shell fluid velocity set by diagonal gaps between adjacent rings, with the radial spacing between the rings decreasing with increasing ring radius. A severe limitation as to the usefulness of this tube array is that the number of rings of tubes is limited in that there is a minium radial separation below which the tubes on the same radial line are too close for proper fabrication. In practice, it has normally been found necessary to have several families of different tubing densities. With different families, tubes nearer the core normally have large ligaments and fewer tubes per ring while the outer tube families have more tubes per ring with small ligaments. Such an array results in the tube packing being significantly less than theoretical optimal. This is a significant draw back in that the density of tube packing is important in a large heat exchanger as it sets the size of the heat exchanger.
An alternative and improved version of radial tube layout is described in U.S. Pat. No. 5,044,431--Cameron wherein it is seen that the gap between the innermost and outermost rings of a family having constant diagonal ligaments is proportional to the radius of curvature of the innermost ring. U.S. Pat. No. 5,044,431, shows and describes a polygonal, including a generally pentagonal, arrangement. However, each tube is set out on arc and each of the tube bundles has a meta center displaced from the center of symmetry of the tube bundles and from the meta centers of the other tube bundles.
Thus, the single family concept can be extended to larger tube counts. This is achieved by splitting the circle into equal sectors each having a pseudo-center located more remote from the tubes than the axis of the shell. A drawback of this design is that open corners are created which are difficult to fill with tubes and thus provide a possible leak path. This disadvantage is reduced in larger heat exchangers.
Tube layout in a radially, symmetrical exchanger having variable radial gaps is also significantly more complicated than a tube layout having straight lines. The multiple family design is again further complicated. There is, therefore, still a need for a tubing layout pattern which provides a compact heat exchanger tubing bundle to provide good heat transfer and yet is simple to fabricate.